This invention relates to a fixture for measuring the contours of a helicopter rotor blade in general and airfoil shape and twist at a particular section and spanwise and chordwise bow along the blade span in particular.
As is well known, to those skilled in the art, small differences in shape and contour can have significant effects in aerodynamic bodies such as wings, propellers, and helicopter rotor blades. Aerodynamicists and manufacturers of such products consume great amounts of time and resources determining the shape or contours of such bodies. In many cases, for example a wing, the contours are predetermined by a variety of techniques and a wind tunnel model is then made in accordance with those contours. It is necessary to inspect the contours of the resultant model so that it may be ascertained if the model was in fact made in accordance with the predetermined contours. To a lesser extent some aerodynamic bodies are built in a wind tunnel by a trial and error method whereupon finding a desired result the contours of the body must then be measured.
In production of an aircraft, such as a helicopter, each rotor blade which is manufactured must be inspected in order to determine whether or not it conforms to the design contours. Particularly, the airfoil sections along a rotor blade span must be manufactured within specific tolerances. Inspection is made more difficult as the airfoil sections vary in shape along the span. Additionally, the twist or angle of each airfoil section with respect to a base or reference airfoil section must be inspected, as must the flapwise and chordwise bow of the rotor blade. Flapwise bow may be defined as the deviation of the rotor blade pitch axis in the plane of rotation of the rotor blade, while chordwise bow may be defined as the deviation of the pitch axis from the plane of rotation.
In actual practice the inspection of aerodynamic bodies in general and helicopter rotor blades in particular have been accomplished with varying degrees of success and efficiency. The airfoil shape at a particular spanwise section has most commonly been inspected through the use of templets having the shape of the desired contour or having a known deviation therefrom. The appropriate templet is placed about the rotor blade at the desired spanwise point and a visual inspection is made to determine if the blade contour matches the templet contour appropriately. This has proven to be a time consuming and inaccurate procedure.
Twist of rotor blades has been determined, also by using templets, by measuring the position of the templet with respect to a horizontal reference plane.
Chordwise and flapwise bow have been measured through the use of a spanwise templet like fixture. The blade is positioned appropriately with respect to the fixture and inspection is accomplished by manually taking measurements along the blade span.
Problems associated with inspecting aerodynamic bodies are recognized in the prior art. U.S. Pat. No. 3,040,437 to Polidor issued June 26, 1962 discloses an apparatus for measuring the airfoil shape of a turbine blade using a multiplicity of pins. U.S. Pat. No. 3,678,585 to Dabrush, et al. issued July 25, 1972 discloses an inspection apparatus for measuring the equivalent of flapwise bow of a turbine engine. U.S. Pat. No. 2,238,782 to Roche issued Apr. 15, 1941 teaches a fixture for measuring twist and width of an aircraft propeller blade with respect to another blade of the same propeller.
There have been many problems associated with various prior art fixtures in that in general they have been inaccurate. Each of the fixtures has measured only a few of the total number of parameters which require inspection, it thus requires a multiplicity of fixtures to inspect a single aerodynamic body such as a rotor blade. Additionally, these prior art fixtures are time consuming to use and usually provide data only at specific spanwise stations along the wing or blade. This is particularly true with respect to those fixtures that measure airfoil contour of a body having different airfoil contours along the span.
The present invention is particularly suited for accurate inspection or measurement of airfoil contours of helicopter rotor blades. The particular embodiment of the invention described herein is a fixture adapted to inspect particular contours of helicopter rotor blades having a constant chord length at locations along its span. The fixture will also measure the twist of any particular airfoil section with respect to a reference airfoil section. Additionally, the fixture will measure the chordwise and flapwise bow of the rotor blade. Although the embodiment herein described has been adapted for inspection of helicopter rotor blades having a constant chord section, it is to be understood that the invention may also be adapted to provide similar measurements with respect to rotor blades having a variable chord and to aerodynamic bodies other than helicopter rotor blades, such as airplane wings, horizontal stabilizers, etc.